Spreading Behavior and Evolution of IMCs During Reactive Wetting of SAC Solders on Smooth and Rough Copper Substrates

The effect of surface roughness of copper substrate on the reactive wetting of Sn-Ag-Cu solder alloys and morphology of intermetallic compounds (IMCs) was investigated. The spreading behavior of solder alloys on smooth and rough Cu substrates was categorized into capillary, diffusion/reaction, and contact angle stabilization zones. The increase in substrate surface roughness improved the wetting of solder alloys, being attributed to the presence of thick Cu₃Sn IMC at the interface. The morphology of IMCs transformed from long needle shaped to short protruded type with an increase in the substrate surface roughness for the Sn-0.3Ag-0.7Cu and Sn-3Ag-0.5Cu solder alloys. However, for the Sn-2.5Ag-0.5Cu solder alloy the needle-shaped IMCs transformed to the completely scallop type with increase in the substrate surface roughness. The effect of Ag content on wetting behavior was not significant.